Mesenchymal stem cells are the formative pluripotential blast cells found inter alia in bone marrow, blood, dermis and periosteum that are capable of differentiating into any of the specific types of mesenchymal or connective tissues (i.e. the tissues of the body that support the specialized elements; particularly adipose, osseous, cartilaginous, elastic, and fibrous connective tissues) depending upon various influences from bioactive factors, such as cytokines. Although these cells are normally present at very low frequencies in bone marrow, the inventors of the present invention have discovered a process for isolating, purifying, and greatly replicating these cells in culture, i.e. in vitro.
The production of hematopoietic cytokines and growth factors by cultured MSCs shows that MSCs support the formation of hematopoietic colonies in co-culture with CD34+ hematopoietic megakaryocytes. Of particular interest to the field of bone marrow transplantation is the role that MSCs may play in the proliferation and differentiation of myeloid precursor cells such as those for megakaryocytes. This could lead to a better understanding of the development of megakaryocytes, which is important since the time to platelet recovery following bone marrow or peripheral blood progenitor transplantation can be very protracted. Cultured MSCs produce GM-CSF, IL-6, IL-11 and thrombopoietin (TPO), cytokines which have been shown to be important for both megakaryocyte and plate production, which is evidence that MSCs function in megakrayocytopoiesis and thrombocytopoiesis. Another link between MSC proliferation and platelet development is that platelet-derived growth factor (PDGF) and serotonin are two major products secreted by platelet that are important to MSC proliferation.
In one aspect, the present invention is directed to human mesenchymal stem cells isolated from a tissue specimen, such as marrow cells, and to the method of their isolation involving separating from the specimen MSCs associated with hematopoietic progenitor cells (such as megakaryocytes). In such a separation process, at least one antibody specific for surface antigens on the megakaryocytes rather than for antigens on MSCs are utilized to obtain an enriched or substantially pure culture of megakaryocytes which have associated therewith MSCs. The MSCs may be recovered by culturing the associated cells under conditions which favor the growth and proliferation of MSCs.